Process to thermally upgrade metal-containing limonite or saprolite ores via magnetic separation and the use of the magnetic concentrate as seeds

ABSTRACT

This invention provides a process to thermally upgrade metal-containing ores comprising the following steps: (1) mixing: (i) ore, (ii) from 4 and up to 15% by weight relative to the ore of reducing agent, (iii) sulphur bearing agent, (iv) metallic-bearing seeding agent, and optionally (v) low temperature binder agent to produce a blend; (2) agglomeration and dry, if required, of the blend formed in step 1 to produce agglomerates; (3) calcination the agglomerates formed in step 2 at reducing atmosphere from a partial pressure of oxygen of Log 10 (pO 2 )=−12 to Log 10 (pO 2 )=−15) and at temperature between 950-1150° C. to produce a liquid metallic phase that growth and concentrate into metallic particles within the agglomerates; (4) cooling the agglomerates after step 3 to ambient temperature in reducing or inert atmosphere; (5) crushing and grinding the calcine produced in step 4 to a size amenable for magnetic separation of metallic particles, typically represented by a p80 equal or below 25 μm; and (6) magnetic concentration of metallic particles by known techniques of magnetic separation including but not limited to magnetic separation by wet or dry means, dewatering and drying. 
     The invention also refers to the magnetic concentrate produced by the process and to the use of the magnetic concentrate to produce a ferronickel or highly metalized nickel containing matte for the production of stainless steel.

FIELD OF THE INVENTION

The present invention refers to a process to thermally upgrade metal-containing ores, particularly nickel-containing lateritic ores, more particularly of limonitic type or a blend of limonitic/saprolitic ores with low or high iron to silica ratio.

The present invention also refers to the magnetic concentrate produced by the process and to the use of the magnetic concentrate.

BACKGROUND OF THE INVENTION

The current processing nickel-containing lateritic ores is carried out by pyrometallurgy or hydrometallurgy means. In both cases, the entire ore needs to be processed since laterite ores are not amenable to concentration by physical means.

Pyrometallurgy mainly treats saprolite (low iron to nickel ratio) and Hydrometallurgy mainly treats limonite (high iron to nickel ratio).

Limonite is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH).nH₂O, although this is not entirely accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the two principal iron ores, the other being hematite.

Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds.

Pyrometallurgy consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring physical and chemical transformations in the materials to enable recovery of valuable metals. The pyrometallurgical processes are generally grouped into one or more of the following categories: drying/calcining/roasting/smelting/converting/refining.

Hydrometallurgy is known as a method for obtaining metals from their ores. It is a technique within the field of extractive metallurgy involving the use of aqueous chemistry for the recovery of metals from ores, concentrates, and recycled or residual materials. The complement to hydrometallurgy is pyrometallurgy. Hydrometallurgy is typically divided into three general areas: leaching; solution concentration and purification; and metal recovery.

The patent U.S. Pat. No. 5,178,666 teaches a thermal upgrading process whereby nickel-containing limonite or limonite/saprolite blends are pelletized with requisite amounts of solid carbon reductant and a sulfur-bearing concentrating agent. This patent does not teach about the use of metallic seeding particles to enhance the recovery and grade of the magnetic concentrate and it is strongly dependent on maintaining a carefully controlled reducing atmosphere.

The new process of the present invention allows for thermal upgrading and pyrometallurgical treatment of metal-containing ores, particularly nickel-containing lateritic ores, more particularly of the limonitic type or a blend of limonitic/saprolitic ores with low or high iron to silica ratio, to produce a calcine containing concentrated metal particles amenable to magnetic separation.

This new process potentially results in overall cost reduction during the treatment of ores by significantly reducing the total volume of material to be treated and to produce valuable metal products.

Although there have been previous attempts to develop either high or low temperature thermal upgrading technologies to process metal-containing ores, there is not any successful commercial implementations of such processes.

The present invention differs from known processes in two main aspects: The first is related to the use of metallic or metal concentrate seeds to enhance and promote metallic particle concentration and growth. The second is that this new process does not require strict atmosphere control during the thermal treatment to achieve successful metallic concentration.

This is accomplished by using adequate amounts and type of reducing agent blended with the ore prior to the thermal treatment.

SUMMARY OF THE INVENTION

The present invention refers to a process to thermally upgrade metal-containing ores comprising the following steps:

-   -   (1) Mixing: (i) ore, (ii) from 4 and up to 15% by weight         relative to the ore of reducing agent, (iii) sulphur bearing         agent, (iv) metallic-bearing seeding agent, and optionally (v)         low temperature binder agent to produce a blend;     -   (2) Agglomeration and dry, if required, of the blend formed in         step 1 to produce agglomerates;     -   (3) Calcination the agglomerates formed in step 2 at reducing         atmosphere (from a partial pressure of oxygen of Log₁₀(pO₂)=−12         to Log₁₀(pO₂)=−15) and at temperature between 950-1150° C. to         produce a liquid metallic phase that growth and concentrate into         metallic particles within the agglomerates;     -   (4) Cooling the agglomerates after step 3 to ambient temperature         in reducing or inert atmosphere;     -   (5) Crushing and grinding the calcine produced in step 4 to a         size amenable (p₈₀<=25 μm) for magnetic separation of metallic         particles; and     -   (6) Magnetic concentration of metallic particles by known         techniques of magnetic separation including but not limited to         magnetic separation by wet or dry means, dewatering and drying.

DETAILED DESCRIPTION OF THE INVENTION

The ores useful in the process of the present invention includes nickel laterites of both limonite and saprolite nature.

The metals within the scope of invention include nickel and iron.

It is a preferred embodiment of the present invention the use of nickel containing lateritic ores.

The nickel containing lateritic ores are, preferably, of the limonitic type or a blend of limonitic/saprolitic ores with low or high iron to silica ratio.

The process of the present invention is applicable to any nickel-containing limonite, saprolite or limonite/saprolite blend of lateritic type of ore that could also contain small amount of other metals such as, but not limited to cobalt or chrome in their elemental or oxide forms.

In addition, the present invention is applicable to the co-processing of lateritic ores in conjunction with nickel-bearing sulphides that could contain any kind of impurities that can be removed by any known method that are technical or economical.

The ore might be prepared to adequate size (below 212 μm) and moisture (10 to 20% by mass) by known means of mineral processing. This pretreatment might involve, but might not be limited to: crushing, screening, desliming, flotations as part of silica rejection process in order to produce ore blend with Fe/SiO₂ ratios above 2.0 g/g.

The suitable amounts of the reducing agent are added to provide necessary reducing conditions during thermal treatment. The suitable amounts of the reducing agent create a locally reducing atmosphere within the agglomerates, making this invention less dependent on careful atmosphere control during calcination.

The reducing agent used in the present invention could be but not limited to solid carbon or liquid hydrocarbon type.

The sulphur bearing agent can be added in amounts from 1 and up to 5 wt % of equivalent contained S relative to the weight of ore.

The sulphur bearing agent could be but not limited to elemental sulphur, nickel-bearing sulphide concentrate, Iron-bearing concentrate, or a blend of sulphides minerals.

The sulphur bearing agent is required to promote growth of valuable metal particle during thermal treatment.

The metallic-bearing seeding agent can be added in amounts as little as 0.1 wt % and as high as 2 wt % relative to the weight of ore in the form of, but not limited to ferronickel particle, ferronickel concentrate, metallic nickel, nickel powder and metallic iron powder.

When present, the low temperature binder agent can be added in amounts from 0 and up to 5 wt. % relative to the mass of ore, to aid in the agglomeration process of the total blend and to provide sufficient strength during handling and processing.

The optional low temperature binder agent could be but not limited to organic binder and silicate binder.

The step (2) of agglomeration and dry might be performed to provide sufficient strength for material handling.

Agglomeration is necessary to create localized reducing conditions. Drying is only necessary if moisture is much higher than 25% by mass. The moisture content of the agglomerates must be in the range of 15 to 25% by mass. The agglomerates that will be calcining at step (3) might be dried or wet.

The dried or wet agglomerates are calcined for at least one hour to a maximum of three hours in contact with reducing atmosphere (equivalent to Log₁₀(pO₂) of −12 to −15) to temperature high enough to produce a liquid metallic phase that growth and concentrate into metallic particles within the agglomerate but not high enough to produce sticking of the agglomerate. Typical temperatures to achieve this purpose are in the range of 950-1150° C. Lower temperatures will not result in the necessary degree of reduction and higher temperatures will result in stickiness and build up problems.

The step of cooling (step 4) is to prevent re-oxidation of metallic particles or partially reduced valuable metals. Cooling rate is adequate to prevent disproportionation of ferrous oxide to form enough amounts of magnetite to be detrimental for the magnetic separation of valuable metal particles.

After the step 4 the cooled agglomerate, which is the calcine produced in step 4, is submitted to mineral processing as crushing and grinding (step 5) to a size amenable for magnetic separation of the metallic particles, typically with a representative p₈₀ equal or lesser than 25 μm.

The product of step 5 is then prepared to produce a magnetic concentrate of valuable metals by known techniques of magnetic separation, including but not limited to magnetic separation by wet or dry means, dewatering and drying.

The product of the step (6) is the magnetic concentrate from which a small portion is recycled as seed material mention in step 1.

The magnetic concentrate, which typically consists of 5 to 15 wt % Ni and with varying Fe/Ni ratios in the range of 1/1 and up to 10/1 by mass with metallic particles ranging in size above 20 μm, produced by the process of the present invention can be used to further produce a ferronickel or highly metalized nickel containing matte for the production of stainless steel.

The new process of the present invention provides unique features:

-   -   Low temperature thermal upgrading of ore that results in high         grade, high recovery metal-containing concentrate;     -   Reduction in operating cost as compared to traditional         pyrometallurgical or hydrometallurgical processes to treat ores         by significantly reducing the volume of material to be treated         in the processing steps;     -   Use of metallic-bearing seeding agent to enhance and promote         growth of valuable metals amenable for magnetic concentration;     -   The required degree of reduction of both nickel and iron in the         agglomerates is controlled by creating a locally reducing         atmosphere within the agglomerates by means of suitable amounts         and type of reducing agent, making the process less dependent on         careful atmosphere control during calcinations and thermal         processing.

It is to be understood that modifications and variations to the proposed invention might be identified and proposed by the skilled in the art to treat metals other than nickel or from ores other than lateritic by their technology. It should be considered that such modifications and variations are considered to be part of the scope of this invention.

The present invention is illustrated by the following examples:

EXAMPLES

In order to demonstrate the new thermal upgrading process for laterite ores and the impact of using metallic seeding particles, bench-scale experiments have been conducted using different laterite ores, S-bearing agents and seeding material.

Pellet batches of approximately 1.2 kg where prepared and split into representative aliquots to be tested at various conditions of temperature profile, reaction time and atmosphere. Calcined pellets were prepared for magnetic separation by grinding the calcine down to p₈₀ of 25 μm and magnetically concentrated by applying 500, 600 and 800 Gauss successively into a stirring flotation cell to produce three magnetic concentrates and a tail. Normally the three magnetic concentrates were analyzed separately but the composite result is being reported here.

In the following tables, all compositions and recoveries are reported in a mass basis.

Chemical analysis of the various raw materials used for this experiments are listed below:

Limonite A: 1.15%Ni, 38.4%Fe, 15.9%SiO₂, 2.7%MgO, 5.4%Al₂O₃, 8.1%Cr₂O₃ Limonite B: 1.59%Ni, 48.5%Fe, 8.1%SiO₂, 1.6%MgO, 4.31%Al₂O₃, 1.8%Cr₂O₃

Sulphide A: 10.01%Ni, 25.2%Fe, 32.6%S, 13.7%Si, 5.1%Mg, 1.7%Cu, 0.3%Co

Concentrate A: 17.3%Ni, 45.4%Fe, 15.3% CaO, 6.3% SiO₂, 5.1%MgO, 1.3%S, 1.5%Al₂O₃ Concentrate B: 17.8%Ni, 55.9%Fe, 7.2% CaO, 7.9% SiO₂, 5.7%MgO, 0.8%S, 0.5%Al₂O₃ Coal A: 68.3% C total, 41% fixed C, 2.1%S, 5.7% Ash

Example 1

The effect of using ferronickel seeds is demonstrated in Table 1. In this case, the pellets were preheated at 600° C. for 1 h and then calcine at 1000° C. for 1 h under reducing atmosphere (CO/CO₂ volume ratio controlled at 2/1). The ferronickel concentrate correspond to magnetic concentrate obtained from ferronickel slag refining process. S source in this case is “Sulphide A” and reductant is “Coal A” added in amounts of 69 and 60 g, respectively

TABLE 1 Experimental conditions and results, Example 1. Feed Blend Magnetic concentrate Ore type Ni Test (1000 g) Seed material recovery, % Ni grade, % 3ii Limonite A none 76.2 9.91 15ii Limonite A Concentrate A (1 g) 92.6 8.6 16ii Limonite A Concentrate A (5 g) 91.4 8.01 17ii Limonite A Concentrate A (10 g) 91.6 8.08 18ii Limonite A Concentrate B (1 g) 90.6 8.52 35ii Limonite B Concentrate A (10 g) 90.5 7.56

Example 2

The effect of using recycled magnetic concentrate as seeding agent from the concentrate generated during the experiments is demonstrated in Table 2.

TABLE 2 Experimental conditions and results, Example 2. Feed Blend Ore type Magnetic concentrate Test (1000 g) Seed material Ni recovery, % Ni grade, % 3ii Limonite A none 76.2 9.91 23ii Limonite A mag. con. from 90.9 8.66 3ii (1 g) 50ii Limonite A mag. con. from 93.6 6.76 23ii (1 g) 59ii Limonite A mag. con. from 93.4 6.38 50ii (1 g)

Example 3

The following table shows the impact of reductant addition on the quality of magnetic concentrate for two cases. The first cases (tests labeled as “ii”) were performed under controlled fully reducing atmosphere, whereas the second case (tests labeled as “v”) where performed by gradually increasing the temperature and reducing potential of the gas (from oxidizing to fully reducing) simulating the change in temperature and atmosphere of a real production kiln. In Example 3, S source corresponds to “sulphide A” and seeding agent is “Concentrate A” both added in amounts of 69 and 1 g, respectively.

TABLE 3 Experimental conditions and results, Example 3. Magnetic Feed Blend concentrate Test Ore type Reductant Ni recovery, % Ni grade, % 15ii Limonite A Coal A (60 g) 92.6 8.6 62ii Limonite A Coal A (70 g) 93.4 9.8 63ii Limonite A Coal A (80 g) 94.3 7.75 53ii Limonite A Coal A (90 g) 91.7 4.93 54ii Limonite A Coal A (120 g) 92.9 3.93 39v Limonite A Coal A (60 g) 91.1 5.1 62v Limonite A Coal A (70 g) 87.1 7.63 63v Limonite A Coal A (80 g) 84.9 7.82 53v Limonite A Coal A (90 g) 97.7 5.66 54v Limonite A Coal A (120 g) 93.3 4.92

As seen from Table 3 above, excess addition of reducing agent to the pellet blend results in an undesirable decrease of Ni grade in the final magnetic concentrate. This is due to the excessive Fe reduction from the ore, having a diluting effect of the concentrate. In addition, although the kiln temperature and atmosphere profile conditions (“v”) are more adverse than a constant fully reducing atmosphere (“ii”) these results demonstrate that the process is robust enough to be carried out under typical kiln conditions, without the need of a carefully controlled reducing atmosphere as required in US patent U.S. Pat. No. 5,178,666.

Example 4

The effect of metallic seed composition is given in Table 4. In this case, the S source (“Sulphide A”) and reductant (“Coal A”) amount used for each test were 69 and 60 g, respectively.

TABLE 4 Experimental conditions and results, Example 4. Feed Blend Magnetic concentrate Ore Ni Test type (1000 g) Seed material recovery, % Ni grade, % 41ii Limonite A Fe powder (1 g) 93.8 8.51 15ii Limonite A concentrate A (1 g) 92.6 8.6 43ii Limonite A Ni powder (1 g) 93.1 6.51 42ii Limonite A Fe powder (10 g) 93.8 8.93 17ii Limonite A concentrate A 91.6 8.08 (10 g) 44ii Limonite A Ni powder (10 g) 93.1 8.89 

What is claimed is:
 1. A process to thermally upgrade metal-containing ores, comprising the following steps: (1) mixing: (i) ore, (ii) from 4 up to 15% by weight relative to the ore of reducing agent, (iii) sulphur bearing agent, (iv) metallic-bearing seeding agent, and optionally (v) low temperature binder agent to produce a blend; (2) agglomeration and dry, if required, of the blend formed in step 1 to produce agglomerates; (3) calcination the agglomerates formed in step 2 at reducing atmosphere (from a partial pressure of oxygen of Log₁₀(pO₂)=−12 to Log₁₀(pO₂) =−15) and at temperature between 950-1150° C. to produce a liquid metallic phase that growth and concentrate into metallic particles within the agglomerates; (4) cooling the agglomerates after step 3 to ambient temperature in reducing or inert atmosphere; (5) crushing and grinding the calcine produced in step 4 to a size amenable (p₈₀≦25 μm) for magnetic separation of metallic particles; and (6) magnetic concentration of metallic particles by known techniques of magnetic separation, including but not limited to magnetic separation by wet or dry means, dewatering and drying.
 2. The process of claim 1, wherein the ores includes nickel laterites of both limonite and saprolite nature.
 3. The process of claim 1, wherein the metal includes nickel and iron.
 4. The process of claim 1, wherein the metal-containing ores is nickel containing lateritic ores.
 5. The process of claim 4, wherein the nickel containing lateritic ores are of the limonitic type or a blend of limonitic/saprolitic ores with low or high iron to silica ratio.
 6. The process of claim 4, applicable to the co-processing of lateritic ores in conjunction with nickel-bearing sulphides that could contain any kind of impurities that can be removed by any known method.
 7. The process of claim 1, wherein includes a pretreatment of the ore in order to prepare the ore to adequate the size (below 212 μm) and moisture (10 to 20% by mass), by known means of mineral processing.
 8. The process of claim 7, wherein this pretreatment might involve, but might not be limited to: crushing, screening, desliming, flotations as part of silica rejection process in order to produce ore blend with Fe/SiO₂ ratios above 2.0 g/g.
 9. The process of claim 1, wherein the reducing agent is solid carbon or liquid hydrocarbon type.
 10. The process of claim 1, wherein the sulphur bearing agent is added in amounts from 1 and up to 5 wt % of equivalent contained S relative to the weight of ore.
 11. The process of claim 1, wherein the sulphur bearing agent is elemental sulphur, nickel-bearing sulphide concentrate, Iron-bearing concentrate, or a blend of sulphides minerals.
 12. The process of claim 1, wherein the metallic-bearing seeding agent is added in amounts as little as 0.1 wt % and as high as 2 wt % relative to the weight of ore.
 13. The process of claim 1, wherein the metallic-bearing seeding agent is ferronickel particle, ferronickel concentrate, metallic nickel, nickel powder and metallic iron powder.
 14. The process of claim 1, wherein the low temperature binder agent is added in amounts from 0 and up to 5 wt.% relative to the mass of ore.
 15. The process of claim 1, wherein the low temperature binder agent is organic binder and silicate binder.
 16. The process of claim 1, wherein agglomerates of step (3) are dried or wet.
 17. The process of claim 16, wherein the dried or wet agglomerates are calcined for at least one hour to a maximum of three hours in contact with reducing atmosphere (equivalent to Log₁₀(pO₂) of −12 to −15) to a temperature are in the range of 950-1150° C.
 18. A magnetic concentrate, characterized by being produced by the process as defined in claim 1, which consists of 5 to 15 wt % Ni and with varying Fe/Ni ratios in the range of 1/1 and up to 10/1 by mass with metallic particles ranging in size above 20 μm.
 19. Use of the magnetic concentrate, produced by the process as defined in claim 1, to produce a ferronickel or highly metalized nickel containing matte for the production of stainless steel. 